1. Field of the Invention
The present invention relates to a drum brake device which is based on feedback control, and is able to constantly produce a stable braking force irrespective of the state of a brake friction surface, and is further adaptable for use in both an anti-lock braking system and a traction control system.
2. Discussion of the Related Art
Drum brake devices are widely employed in vehicle braking systems. A major reason for the drum brake device's use is its ability to automatically amplify a braking force (referred to as a self-amplifying function), as is observed in the uni-servo or duo-servo brake devices. This self-amplifying ability lessens the required brake operation force (e.g., a brake pedal depression force). However, this self-amplifying operation of the drum brake device is unstable. A proportional constant, which defines a relationship between a pedal depressing force and a final braking force, is very sensitive to the friction coefficient of the interface between the brake drum and the brake shoe. In cases where the inner surface of the brake drum is rusted after the vehicle travels in the rain, the wheels will be abruptly locked by applying an extremely small depression to the brake pedal.
To remove this defect, it is essential that the drum brake device have the ability to constantly produce a stable braking force at a predetermined boosting ratio in response to a brake operation force, even if the frictional characteristic of the interface between the brake drum and the brake shoe varies. One solution to this problem was proposed in JP-B-2-46424. The technique of this publication will be described with reference to FIGS. 2 and 3.
As shown, a drum brake device 1 is made up of a pair of brake shoes 3 and 4, a wheel cylinder 6, a link member 8, a master cylinder 12, and a fluid passage 13. In the drum brake device 1, the wheel cylinder 6 is improved to produce a stable braking force. The brake shoes 3 and 4 are oppositely disposed within an inner space of a cylindrical brake drum 2. The wheel cylinder 6, disposed between the opposed ends of brake shoes 3 and 4, is used for expanding the ends of the brake shoes 3 and 4 so that they come into engagement with the inner surface of the brake drum. The link member 8 mutually links the other ends of the brake shoes 3 and 4, and receives an anchor reaction force of one of the brake shoes 3 and 4 and transmits it to the other brake shoe. The master cylinder 12 generates a hydraulic braking pressure that corresponds in magnitude to a brake operation force F1 (a depression force applied to a brake pedal 10). The hydraulic braking pressure generated by the master cylinder 12 is introduced into the wheel cylinder 6 through the fluid passage 13.
Details of the wheel cylinder 6 are illustrated in FIG. 3. As shown, a cylinder body 17 includes a first cylinder 17a and a second cylinder 17b. These cylinders (17a and 17b) are integrally formed, with the former (17a) being located below the latter (17b). The first cylinder 17a contains a slidable drive piston 15. The second cylinder 17b contains a slidable control piston 16. A hydraulic braking pressure is transmitted from the fluid passage 13 to a pressure chamber 20 via a control chamber 19. The control chamber 19 is formed in the second cylinder 17b while the pressure chamber 20 is formed in the first cylinder 17a. The tip of the drive piston 15 contacts the primary shoe 3, and presses the primary shoe 3 against the brake drum 2 by a thrust force P1. Thrust force P1 corresponds to the hydraulic braking pressure supplied to the pressure chamber 20.
The tip of the control piston 16, which is in contact with the end of the secondary shoe 4, receives an anchor reaction force P2 from the secondary shoe 4 while the base end of the control piston 16 receives a hydraulic braking pressure supplied to the control chamber 19. When an urging force corresponding to the anchor reaction force P2 exceeds an urging force P3 caused by the hydraulic braking pressure, the control piston 16 is displaced toward the control chamber 19. Further, a valve body 24 is provided within the control chamber 19. The valve body is used for opening and closing a communicating passage 22 which communicates the control chamber 19 with the pressure chamber 20. When the control piston 16 is displaced toward the control chamber 19, the communicating passage 22 is closed with the valve body 24.
When the urging force P3 (caused by the hydraulic braking pressure from the master cylinder 12) is imparted or input to the control piston 16, and the anchor reaction force P2 is varied to a force defined by a predetermined boosting ratio, the control piston 16 is displaced toward the control chamber 19 to stop the supply of the hydraulic braking pressure to the pressure chamber 20. As a result, the thrust force P1 of the drive piston 15 is kept constant, a further increase of the anchor reaction force is prevented, and the braking force is stabilized.
In the conventional drum brake device mentioned above, the mechanism for controlling the anchor reaction force is incorporated into the wheel cylinder 6. However, this type of drum brake device has the following problem. The structure of the brake device increases the size of the wheel cylinder 6. The increase of the cylinder size makes it difficult to assemble the wheel cylinder 6 into the brake device of a small-size vehicle that has a brake drum with a small inner space. Additionally, the increased cylinder size further increases the weight of the brake device with the wheel cylinder assembled thereinto. For this reason, the conventional drum brake device which employs the unique mechanism for achieving braking force stabilization has found limited use in small-size vehicles. It is almost impossible to apply a common conventional drum brake device to various types of vehicles. Specifically, vehicles differ in body weight and in anchor reaction forces of the drum brake devices assembled thereinto. Accordingly, an object of the present invention is to provide a drum brake device which produces a stable braking force at a predetermined boosting ratio in response to a brake operation force input thereto, even if the frictional characteristics of the interface of the brake drum and the brake shoe varies. Additionally, the aforementioned object should be realized without increasing the device size. Furthermore, the drum brake device should be suitably applicable to a small-size vehicle having a brake drum with a small inner space.